Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability (for two-tailed, .05 and .01) and get the respective Z scores.
90% = Mean ± Z(SD)
98% = mean ± Z(SD)
After the calculations, you should be able to answer why.
The average one-way commute time for a random sample of 256 area residents of a city is 90 minutes, with a standard deviation of 24 minutes.
Construct a 90% confidence interval for the mean commute time of all residents.
Construct a 98% confidence interval for the mean commute time of all residents.
Which interval is larger? Why?
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